1. Field of the Invention.
This invention relates to automatic temperature control systems for controlling the temperature within an enclosure and more particularly such control systems of the type used to automatically control the temperature within the passenger compartment of automotive vehicles.
2. Description of the Prior Art.
Automatic temperature control systems for automotive vehicles commonly utilize a control scheme in which air is circulated through a ducting system by a blower, and first cooled by passing the air through an evaporator coil of an air conditioner, the air then being circulated to a diverter box in which the cooled air is proportioned between a bypass duct passage into the interior of the passenger compartment and a heater passage in which is located the heater core. The relative proportion of air moved through the bypass or heater duct passages thus controlling the temperature of the air circulated into the passenger compartment.
This proportioning is controlled by means of a blend air door which is pivotally mounted to divert a portion of the cool air to one or the other of the bypass or heater duct passages. The position of the blend air door in turn is controlled by means of a vacuum operated actuator which causes the blend air door to assume various positions in correspondence with the level of vacuum pressure existing within the vacuum operated actuator.
The pressure level is determined by the temperature controls of the system acting on a vacuum modulator device which modulates vacuum pressure in correspondence with a temperature error signal, i.e., the difference between a manually selected temperature level and the sensed temperature level. This correspondence is such that with the increasing magnitude of the error signal, a correspondingly great change in vacuum pressure is produced to divert a relatively greater proportion of air in either the bypass of the heater duct. The sense of the error signal controls the sense of the pressure change and the direction of movement of the blend air door.
For example, if the temperature in the passenger compartment is greater than that selected, the vacuum modulator device is adjusted to cause the blend air door to divert a greater proportion of the cooled air into the diverter box bypass duct. Similarly, if the temperature level in the engine compartment falls below the selected level, the vacuum modulator pressure is varied by the system controls to cause the vacuum operated actuator to move the blend air door to a position in which a greater proportion of air is diverted past the heater core. Since the error signal relied on for automatic control of the position of the blend air door is basically a thermal signal, i.e., the difference between the selected and sensed temperature levels, the system must be stabilized due to the time lags inherent in a temperature control system. Particularly is this so in these systems in which the change in position of the blend air door lags considerably the change in temperature of the passenger compartment.
Commonly, such systems are stabilized by the use of a force balancing system such as a diaphragm in which a temperature error responsive control member acts against the forces generated by the pressure developed in the vacuum modulator. A typical example of such a system is disclosed in the Amano et al U.S. Pat. No. 3,877,638 in which a heated bimetal element is moved in response to the generation of an error signal. The force balancing of the heated bimetal against the diaphragm subjected to the regulated vacuum rapidly stabilizes the system.
A substantial drawback to these systems results from the absence of control by the difference between the sensed and selected temperature levels, and the use of the force balance stabilization in the vacuum modulator. This drawback is in the considerable variations in the actual temperature achieved by the system controls for a given selected temperature level with variations in various system and external conditions such as ambient air, pressure and temperature engine vacuum levels, etc.
It thus has been heretofore proposed to stabilize the control system by utilizing a position feedback potentiometer directly associated with the blend air door, which feedback signal is used to null the temperature error signal. It has been discovered that this approach improves the system's response in that the stabilization to the error signal is more quickly achieved, but more importantly, it has been discovered that the correspondence between the selected temperature level and the temperature level actually maintained in the passenger compartment must more closely correspond to each other.
Such a system is disclosed in the Weaver et al U.S. Pat. No. RE 27,699 which discloses a position feedback potentiometer driven by mechanical movement of the door actuator for the blend air door.
While such arrangements have been successful, space limitations sometimes preclude the installation of such feedback potentiometers and other devices, and further increase the mechanical complexity of the system.
Another disadvantage of the force balance systems of Amano et al is that the pressure changes including the demands of the actuator cannot be met with the full application of the vacuum sources, since only a metering communication between the source and the vacuum regulator is provided in that system.
This is contrasted with the approach of the Weaver et al patent in which full communication of the source is created until the correct position of the actuator is achieved. Obviously, the response of the system will be faster with the approach of the Weaver et al patent.
It is, therefore, an object of the present invention to provide an improved automatic temperature control system of the type described in which a control system stabilizing feedback signal is provided in which the temperature level achieved by the control system is in close correspondence with the selected level.
It is yet another object of the present invention to provide such an automatic temperature control system in which such stabilizing feedback signal is provided which does not involve a force balancing system with the temperature controlled output member.
It is still another object of the present invention to provide such automatic temperature control system in which the feedback signal does not require a mechanical connection with the blend air door or actuator movable members.
It is yet another object of the present invention to provide feedback signals by relatively simple reliable means for achieving the rapid response and constant temperature control desirable in such control systems.
Another object of the present invention is to provide such system in which the advantages of full source communication with the actuator are also obtained.